This invention relates to the field of processes for producing wholly aromatic polyesters, or polyarylates, especially such polymers having low color.
Aromatic polyesters have many well-known uses. One use for these materials is as a transparent material; ideally, the material should be color-free as well. An amorphous polymer is required for high transparency materials because crystalline polymers have poor transparency.
Amorphous polyesters are commonly made by melt polymerization, which requires high temperatures. Unfortunately, these high temperatures cause undesirable coloration of the material if maintained for a long period of time, but if the polymerization is abbreviated to avoid significant color formation the polymer will have a low viscosity and molecular weight and will not exhibit acceptable physical properties.
U.S. Pat. No. 3,684,766 discloses a process for enhancing the inherent viscosity of bisphenol polyesters wherein isophthalic acid and terephthalic acid units are copolymerized with bisphenol A and the resulting polyester is contacted with a crystallizing medium and heated to produce a polyester having an inherent viscosity of 0.5 or more. This reference teaches how to make bisphenol A polyesters containing terephthalic acid and isophthalic acid monomers in a mole ratio of between 50:50 and 70:30, respectively, but teaches that a polymer having a higher proportion of isophthalic acid cannot be crystallized sufficiently to produce a high viscosity polymer. U.S. Pat. No. 3,780,148 is a continuation-in-part of the '766 patent.
A process for preparing partially aromatic polyesters is disclosed in U.S. Pat. No. 3,972,852: aromatic dicarboxylic acids and aliphatic diols are polymerized in two stages, in a melt and then in the solid state. The polymer may be heat treated or treated with a crystallizing solvent between the melt polymerization step and the solid state polymerization step.
U.S. Pat. No. 4,314,051 describes a process for preparing polyarylates comprising forming a crystalline prepolymer and then heating below the prepolymer melting point to form a crystalline polyarylate having a viscosity of 0.45 to 1.2 dl/g.
A wholly aromatic copolyester and a process therefor is taught by U.S. Pat. No. 4,436,894. In this process, a copolyester composed primarily of isophthalic acid units and hydroquinone units is made by polycondensing the monomers under heat until the viscosity reaches 0.2 and then continuing the polymerization in a molten state at a temperature below the crystalline melting point of the polymer.
U.S. Pat. No. 4,994,546 teaches a process for preparing bisphenol polyarylates wherein a prepolymer is formed from bisphenol A and at least one aromatic dicarboxylic acid and the prepolymer is polycondensed under agitation to form a powdery solid which is further polymerized until the desired molecular weight is reached.
Japanese Pat. No. 53054295 (1978) discloses polyesters comprising mainly the ester unit of aromatic dicarboxylic acids and 2,2-bis(4'-hydroxyphenyl)propane produced by allowing amorphous polyester to stand in contact with a crystallization media until the surface of the polyester particles is partly crystallized and then polymerizing further in the solid state.